Kinetic control of dissolved phosphate in natural rivers and estuaries: A primer on the phosphate buffer mechanism

The primary mode of interaction of dissolved phosphate with fluvial inorganic suspended particles is via a reversible two‐step sorption process. The first step, adsorption/desorption on surfaces, has fast kinetics (minutes–hours). The second step, solid‐state diffusion of adsorbed phosphate from the surface into the interior of particles, has slow kinetics (days–months) and is dependent on the time history of the previous surface sorption and the chemistry of the solid diffusional layer. Natural clay particles with a surficial armoring of reactive iron and aluminum hydroxyoxides resulting from chemical weathering of rocks and soils have a high capacity for absorbing phosphate in the second step and for maintaining low “equilibrium phosphate concentrations” in solution. Extrapolation of laboratory sorption and extraction experiments with natural soils and suspended sediments to the environment suggests that the phosphate concentrations of unperturbed turbid rivers (SPM > 50 mg liter −1 ) are controlled near the dynamic equilibrium phosphate concentration of their particles (EPC 0 = 0.2–1.5 µ M) and that fluvial suspended particles “at equilibrium” contain up to 10 µ mol‐P g −1 that is desorbable. Release of this phosphate from particles entering the sea produces the characteristic shape and magnitude of input profiles of dissolved phosphate observed in unperturbed estuaries. On a global scale, fluvial particulates could transport from 1.4 to 14 × 10 10 mol yr −1 of reactive phosphate to the sea, some 2–5 times more than that in the dissolved load alone.